Error measurement in time of flight tests

I build a potato cannon with my friend and we were having fun shooting it safely in a clearing. I decided to shoot is straight up and measure the time it is in air. Out of three measurements I got 9, 9, and 7 seconds which is a long time. Taking and average and Using 2 dim eqs I got 2v=at, v=40.875 for initial and final velocity, Then using vf^2=Vo^2 +2ax I got a height of 170m or about 558 feet. How much of a error could I expect to receive. It was at about 90 degrees and I figure if the wind is blowing horizontal then there would be no error considering that it wouldn't affect gravity. Thanks.

Velocity would be fairly accurate as to the height you'd be better doing some basic trig to figure out the height since you wouldn't really have constant acceleration if it hits it's terminal velocity.

I don't think it hit terminal velocity at all. Also, the day had relatively not wind at all, but that does not mean that there was wind at higher altitudes which there almost always is. 90mi/hr, it a large number and for this potato cannon, I'm just making sure it's near accurate. Keep in mind the measurements of time was taken from at average and is near exact. Thanks